Apparatus and method for optical component alignment

Abstract

Apparatus and methods for aligning optical components, such as beam splitters, in an optical pulse stretcher align the entrance spot of a beam passing through a portion of the optical beam splitter with the coincident entrance spot of a beam split from a retro-reflected input beam on a target spot. Apparatus and methods for aligning retroreflectors to facilitate proper beam alignment are also disclosed. Fluorescent materials can be used to visualize the beam entrance spot.

Description

Technical Field

[0001] (Cross - reference to related applications)

[0001] This application claims priority to U.S. Application No. 63 / 126,067, entitled "APPARATUS FOR AND METHOD OF OPTICAL COMPONENT ALIGNMENT FIELD", filed on December 16, 2020, which is hereby incorporated by reference in its entirety.

[0002]

[0002] This application relates to systems and methods for aligning optical components, for example, used in a lithographic apparatus, and more particularly to components of an optical pulse stretcher useful for stretching the pulses of the output of a laser source.

Background Art

[0003]

[0003] A lithographic apparatus applies a desired pattern to a substrate, such as a wafer of semiconductor material, usually to a target portion of the substrate. A patterning device, such as a mask or reticle, can be used to generate the circuit pattern to be formed on individual layers of the wafer. The transfer of the pattern is usually effected by imaging onto a layer of radiation - sensitive material (resist) provided on the substrate. Generally, one substrate includes adjacent target portions to which patterns are sequentially applied.

[0004]

[0004] A lithographic apparatus includes a so - called stepper, in which each target portion is irradiated by exposing the entire pattern to the target portion in one go, and a so - called scanner, in which each target portion is irradiated by scanning the pattern with a radiation beam in the scan direction while synchronously scanning the substrate parallel or antiparallel to a given direction (the "scan" direction). Also, it is possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate. For simplicity, in this specification, both the stepper and the scanner are simply referred to as a scanner.

[0005]

[0005] The light source used to illuminate the pattern and project it onto the substrate can be one of several configurations. Deep ultraviolet excimer lasers commonly used in lithography systems include krypton fluoride (KrF) lasers with a wavelength of 248 nm and argon fluoride (ArF) lasers with a wavelength of 193 nm. The laser source may include an optical pulse stretcher to extend the output pulses of a high-power gas discharge laser system.

[0006]

[0006] New requirements for lithography scanner performance necessitate longer pulse lengths. This is measured by the time integral square (TIS) of the pulse length. For example, a longer TIS is required to improve the edge placement error (EPE) of chip features. To achieve the desired TIS, an optical pulse stretcher (OPuS) is used to stretch the pulse. Increasing the TIS requires a larger OPuS. As the size of the OPuS increases, it becomes even more important that the components of the OPuS are properly optically aligned.

[0007]

[0007] A conventional method for aligning components of an OPuS involves opening the OPuS casing and physically positioning a target card near the optical surface of the component to be aligned. The incoming beam is then aligned based on the incident position (or footprint) of the beam on the target card. The target card is then shifted to the next optical component that requires alignment, and this is continued until all optical components that require alignment are properly aligned. As used herein, “ray” and “beam” have the same meaning.

[0008]

[0008] This alignment process, sometimes called open beam paper target alignment, requires opening the sealed laser casing to place the card. This carries the risk of open beam operation and must be performed with extreme care to avoid exposing the skin of the person performing the alignment to UV radiation. It is also time-consuming and requires a large amount of manual work. This can expose the optical surface being aligned to contamination, potentially shortening the optical lifetime of critical optical components. Furthermore, due to the complexity of setting the beam alignment target and the subjective judgment of beam position, achieving accurate alignment is difficult.

[0009]

[0009] The alignment process also includes aligning the beam splitters within the OPuS. This process involves several steps, but if the beam "falls off of" one of the mirrors during the alignment process, i.e., if it does not hit the mirror, some of those steps may need to be repeated. This can significantly increase the time required to perform the alignment.

[0010]

[0010] In this situation, the need for the subject matter to be disclosed arises. [Overview of the project]

[0011]

[0011] The following provides a brief summary of one or more embodiments in order to provide a basic understanding of these embodiments. This summary is not intended to be an extensive overview of all conceivable embodiments, nor to identify important or essential elements of all embodiments, nor to define the scope of any or all embodiments. Its sole purpose is to present some concepts of one or more embodiments in a simplified form as preparation for the more detailed descriptions to be presented later.

[0012]

[0012] In one embodiment, a camera system is used to monitor the beam position on OPuS components such as mirrors in order to facilitate proper beam alignment. In some embodiments, an image integration optics system is used to integrate images from multiple optical features, such as both the left and right mirror banks, and present these images to the camera system simultaneously. This simplifies the design and avoids the cost of having multiple cameras to inspect features separately. In one embodiment, the camera system is placed outside the purge volume of the OPuS casing to reduce the risk of open beam operation and avoid purge breaking. As used herein, the term "camera" is intended to encompass any device, system, or apparatus for capturing (transforming) an image.

[0013]

[0013] According to one embodiment, during alignment, the incident position of the beam, as revealed by the "fluorescence footprint," is positioned to coincide with or overlap with the alignment feature of the component being aligned. For example, if the optical component is a dichroic mirror in OPuS, the alignment feature can be placed on the rear support plate via the dichroic mirror to facilitate beam positioning.

[0014]

[0014] Accordingly, according to some embodiments, the use of the camera system minimizes the amount of open beam movement required during alignment. This greatly improves the safety of the alignment procedure. The non-contact nature of this alignment system also reduces the risks inherent in handling direct proximity optical components, including the risk of surface contamination. Furthermore, since the system does not require the opening and purging of the casing and manual target positioning is not performed, the field service time of the OPuS module can be significantly reduced.

[0015]

[0015] According to one embodiment, an optical component is disclosed. This optical component comprises a sealed enclosure including a window transparent to visible light; a first optical feature positioned at a first location within the enclosure; a second optical feature positioned at a second location within the enclosure; and an image merging module positioned to receive first optical feature light from the first optical feature and second optical feature light from the second optical feature, and adapted to redirect the first optical feature light and second optical feature light through the window to form an image from the first optical feature light juxtaposed with an image from the second optical feature light. The optical component may be an optical pulse stretcher. The first optical feature may include a first mirror, and the second optical feature may include a second mirror. The first mirror may include a first concave dichroic mirror, and the second mirror may include a second concave dichroic mirror. The first optical feature and the second optical feature may be positioned substantially symmetrically with respect to the image merging module.

[0016]

[0016] According to another embodiment, the image integration module may include a first reflective surface arranged to redirect light from a first optical feature, and a second reflective surface arranged to redirect light from a second optical surface. The first reflective surface may include a first prism reflective surface of a first prism, and the second reflective surface may include a second prism reflective surface of a second prism. The image integration module may include a prism having a first reflective surface directed toward the first optical feature and a second reflective surface directed toward the second optical feature. The image integration module may include two flat, beveled mirrors.

[0017]

[0017] According to another embodiment, the optical component may further include a third optical feature positioned at a third location within the casing and a fourth optical feature positioned at a fourth location within the casing. The image integration module is arranged to receive third optical feature light from the third optical feature and fourth optical feature light from the fourth optical feature, and is adapted to combine and redirect the third and fourth optical feature light through a window to form an image from the third optical feature light adjacent to the image from the fourth optical feature light.

[0018]

[0018] According to another embodiment, the optical component may further include a camera system positioned to receive first optical feature light and second optical feature light through a window. The camera system may include a lens system positioned to receive first optical feature light and second optical feature light through a window, and a camera positioned to receive first optical feature light and second optical feature light from the lens system. The optical component may further include a folding mirror optically positioned between the image integration module and the window to bend the optical paths of the first optical feature light and second optical feature light.

[0019]

[0019] According to another embodiment, at least one of the first optical feature and the second optical feature may be adjustable, and an actuator mechanically coupled to at least one of the first optical feature and the second optical feature may be included for adjusting the orientation of at least one of the first optical feature and the second optical feature.

[0020]

[0020] According to another embodiment, the first optical feature may include a first fluorescent material and a first alignment feature, and the second optical feature may include a second fluorescent material and a second alignment feature. The first optical feature may include a first mirror and a first mirror support, which include a first substrate transparent to visible light and a first reflective coating that reflects UV radiation, and the second optical feature may include a second mirror and a second mirror support, which include a second substrate transparent to visible light and a second reflective coating that reflects UV radiation. The first mirror support may include a first alignment feature on the front surface of the first mirror support, and the second mirror support may include a second alignment feature on the front surface of the second mirror support. The first alignment feature may correspond to the position of the aligned beam footprint on the first mirror, and the second alignment feature may correspond to the position of the aligned beam footprint on the second mirror.

[0021]

[0021] According to another embodiment, the first optical feature may include a first reflective coating comprising a first fluorescent material, and the second optical feature may include a second reflective coating comprising a second fluorescent material. The first fluorescent material may be provided on the rear surface of the first substrate, and the second fluorescent material may be provided on the rear surface of the second substrate. The first fluorescent material may be provided on the front surface of the first mirror support, and the second fluorescent material may be provided on the front surface of the second mirror support. The first mirror support may include the first fluorescent material, and the second mirror support may include the second fluorescent material.

[0022]

[0022] According to another embodiment, an optical component is disclosed. This optical component comprises a sealed enclosure including a window transparent to visible light; a first optical feature positioned within a first field of view within the enclosure; a second optical feature positioned within the first field of view within the enclosure; and an image integration module positioned to receive first field of view light from the first field of view and adapted to combine the first field of view light and redirect it through the window. The image integration module may include a planar mirror surface which is inclined by an angle θ given below with respect to a line passing through the center of the planar mirror surface and substantially parallel to the first field of view.

[0023]

[0023]

number

[0024]

[0024] Here, h is the height of the first field of view, d is the vertical distance between the center of the first field of view and the center of the planar mirror, and s is the horizontal distance between the center of the first field of view and the center of the planar mirror.

[0025]

[0025] According to another embodiment, a method for aligning a plurality of optical features arranged within a sealed enclosure having a window is disclosed. The method includes combining light from each optical feature to generate a combined optical signal, inducing the combined optical signal to the outside of the enclosure through the window, and aligning at least some of the plurality of optical features based at least partially on the combined optical signal. The method may further include, after inducing the combined optical signal to the outside of the enclosure through the window, imaging the combined optical signal using a camera system positioned outside the sealed enclosure and arranged to receive the combined optical signal. Each of the plurality of optical features may include an alignment feature and a fluorescent material arranged to generate a visible footprint of a UV beam incident on the optical feature. The method may further include exposing each optical feature to a UV emission beam and generating an illumination footprint of the UV emission beam on each optical feature before combining light from each optical feature to generate a combined optical signal. Aligning at least some of a group of optical features based at least partially on a combined optical signal may include aligning at least some of a group of optical features based at least partially on the positional relationship between the illumination footprint and an alignment feature at each optical feature. Alignment may include adjusting one or more of the group of optical features. Adjusting one or more of the group of optical features may include manually operating one or more actuators mechanically coupled to each of the one or more of the group of optical features. Adjusting one or more of the group of optical features may include supplying signals to operate one or more motor-driven actuators mechanically coupled to each of the one or more of the group of optical features. Adjusting one or more of the group of optical features may include adjusting the orientation of one or more of the group of optical features.

[0026] According to another aspect of the embodiment, a method for aligning a beam splitter in a confocal optical pulse stretcher is disclosed. The confocal optical pulse stretcher includes at least a first concave mirror and a second concave mirror that is confocal with the first concave mirror. The beam splitter is optically disposed (i.e., on the optical path therebetween) between a first spot on the first concave mirror and a second spot on the second concave mirror. The beam splitter is further arranged to split a first incoming beam traveling in a first direction into a first beam portion directed to the first spot and a second beam portion continuing to travel in the first direction towards the retroreflector. The first beam portion passes through an optical path within the confocal optical pulse stretcher, and this optical path includes a segment between the second spot and the beam splitter. The first beam portion strikes the second spot of the second mirror. The method includes retroreflecting the second beam portion along a return path in a second direction opposite to the first direction back to the beam splitter, splitting the retroreflected second beam portion into a beam traveling from the beam splitter to the second spot and a third beam portion continuing to travel in the second direction, and aligning the split and retroreflected second beam portion to be incident on the second spot of the second mirror. The method may further include applying a part of the third beam portion to a fluorescent screen to generate an image and aligning the retroreflector at least partially based on a determination of whether the image has a predetermined feature. Applying at least a part of the third beam portion to a fluorescent screen to generate an image may include reflecting at least a part of the third beam portion onto the fluorescent screen using a reflecting element having an aperture. Determining whether the image has a predetermined feature includes determining whether the image includes light symmetrically reflected from the periphery of the aperture.

[0027]

[0027] According to another embodiment, a confocal optical pulse stretcher is disclosed. This confocal optical pulse stretcher comprises a first concave mirror, a second concave mirror confocal with the first concave mirror, a retroreflector, and a beam splitter optically positioned between the first concave mirror and the second concave mirror, the beam splitter further positioned to split a first beam traveling in a first direction into a first beam portion guided to the first concave mirror and a second beam portion traveling in the first direction. The first beam portion travels through a closed optical path in the confocal optical pulse stretcher, the optical path including a segment between the second concave mirror and the beam splitter. The retroreflector is positioned to backreflect the second beam portion to produce a backreflected beam traveling in a second direction opposite to the first direction. The beam splitter is further positioned to split the backreflected beam into a third beam portion that enters the second concave mirror through the segment. A retroreflector may include a mirror with an adjustable tip and tilt. A retroreflector may include a folding mirror and a fixed mirror, both with adjustable tips and tilts, where the folding mirror is positioned to receive a second beam portion and redirect it toward the fixed mirror, and the fixed mirror is configured to reflect the second beam portion toward the folding mirror. A confocal optical pulse stretcher may further include a reflective element having an aperture positioned to receive and redirect back-reflected light that has passed through a beam splitter and travels in a second direction opposite to a first direction, and a fluorescent screen positioned to receive the redirected light. The reflective element may have an aperture and be oriented at an angle to the light passing through the beam splitter. A UV reflective element may have a reflective surface positioned to redirect the light.

[0028] According to another aspect of the embodiment, an apparatus for facilitating the alignment of a beam splitter in a confocal optical pulse stretcher is disclosed. The confocal optical pulse stretcher includes at least a first concave mirror and a second concave mirror confocal with the first concave mirror. The confocal optical pulse stretcher further includes a retroreflector. The beam splitter is optically disposed between the first concave mirror and the second concave mirror. The beam splitter is further arranged to split a first beam into a first beam portion guided to the first concave mirror and a second beam portion traveling in a first direction toward the retroreflector. The retroreflector is arranged to retroreflect the second beam portion to generate a retroreflected beam. The first beam portion passes through an optical path in the confocal optical pulse stretcher, and this beam path includes a segment between the second concave mirror and the beam splitter. The beam splitter is further arranged to split the retroreflected beam into a first portion traveling to the second mirror and a second portion passing through the beam splitter. The apparatus includes a reflecting element having an aperture arranged to receive the second portion of the retroreflected beam light passing through the beam splitter and to deflect this light, and a fluorescent screen arranged to receive the deflected light. The reflecting element is oriented at an angle with respect to the second portion of the retroreflected beam passing through the beam splitter and may have an aperture. The reflecting element may have a reflecting surface arranged to deflect the second portion of the retroreflected beam.

[0029]

[0029] Further features and advantages of the present invention and the structure and operation of various embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be noted that the present invention is not limited to the specific embodiments described herein. Such embodiments are described herein for illustrative purposes only. Those skilled in the art will readily conceive of further embodiments based on the teachings contained herein.

Brief Description of the Drawings

[0030]

[0030] The accompanying drawings incorporated herein and forming part thereof illustrate the methods and systems of embodiments of the present invention as examples, not as limitations. The drawings, together with the detailed description, further serve to illustrate the principles of the methods and systems presented herein and to enable those skilled in the art to construct and use these methods and systems. In the drawings, the same reference numerals represent identical or functionally similar elements.

[0031] [Figure 1]

[0031] A schematic diagram, not to a specific scale, illustrating the overall broad concept of a photolithography system according to one aspect of the subject matter disclosed. [Figure 2]

[0032] This is a schematic diagram, not to a specific scale, illustrating the overall broad concept of a laser system used in a lithography system according to one aspect of the disclosed subject matter. [Figure 3]

[0033] This is a diagram of an optical pulse stretcher according to one aspect of the disclosed subject matter. [Figure 4]

[0034] This figure shows various optical paths within an optical pulse stretcher according to one aspect of the disclosed subject matter. [Figure 5A]

[0035] This figure shows various optical paths within an optical pulse stretcher according to one aspect of the disclosed subject matter. [Figure 5B]

[0035] This figure shows various optical paths within an optical pulse stretcher according to one aspect of the subject matter disclosed. [Figure 6]

[0036] This is a partial perspective view showing the arrangement of mirrors on one side of an optical pulse stretcher according to one aspect of the subject matter disclosed. [Figure 7]

[0037] This figure shows a conventional method for aligning optical features within an optical component. [Figure 8A]

[0038] This figure shows a system for aligning optical features according to one aspect of the disclosed subject matter. [Figure 8B]

[0038] This figure shows a system for aligning optical features according to one aspect of the subject matter disclosed. [Figure 9A]

[0039] This figure shows a system for aligning optical features according to one aspect of the disclosed subject matter. [Figure 9B]

[0040] This figure shows the juxtaposition of images in a system for aligning optical features, according to one aspect of the disclosed subject matter. [Figure 10]

[0041] This figure shows the conditions for achieving the maximum field of view according to one aspect of the disclosed subject matter. [Figure 11A]

[0042] An alternative component of the image integration module according to one aspect of the disclosed subject matter is shown. [Figure 11B]

[0042] An alternative component of an image integration module according to one aspect of the disclosed subject matter is shown. [Figure 11C]

[0043] This figure shows the effect on the field of view of the arrangement of components within an optical image integration module, according to one aspect of the disclosed subject matter. [Figure 12A]

[0044] This figure shows a system for aligning optical features according to one aspect of the disclosed subject matter. [Figure 12B]

[0045] This figure shows the juxtaposition of images in a system for aligning optical features, according to one aspect of the disclosed subject matter. [Figure 13A]

[0046] This shows an aspect of the structure of an optical feature according to an aspect of the disclosed subject matter. [Figure 13B]

[0046] An aspect of the structure of an optical feature according to an aspect of the disclosed subject matter is shown. [Figure 13C]

[0046] An aspect of the structure of an optical feature according to an aspect of the disclosed subject matter is shown. [Figure 13D]

[0046] An aspect of the structure of an optical feature according to an aspect of the disclosed subject matter is shown. [Figure 14A]

[0047] This shows an aspect of the structure of an optical feature according to an aspect of the disclosed subject matter. [Figure 14B]

[0047] An aspect of the structure of an optical feature according to an aspect of the disclosed subject matter is shown. [Figure 14C]

[0047] An aspect of the structure of an optical feature according to an aspect of the disclosed subject matter is shown. [Figure 14D]

[0047] An aspect of the structure of an optical feature according to an aspect of the disclosed subject matter is shown. [Figure 15A]

[0048] This shows the relative positioning of the UV footprint and alignment marks before the alignment process, according to one aspect of the disclosed subject matter. [Figure 15B]

[0048] The relative positioning of the UV footprint and alignment marks after the alignment process is shown according to one aspect of the subject matter disclosed. [Figure 16]

[0049] This is a flowchart showing a method for aligning optical features of an optical component according to one aspect of the disclosed subject matter. [Figure 17]

[0050] This figure shows various optical paths within an optical pulse stretcher according to one aspect of the disclosed subject matter. [Figure 18]

[0051] This figure shows various optical paths within an optical pulse stretcher according to one aspect of the disclosed subject matter. [Figure 19]

[0052] This is a flowchart showing a method for aligning optical features of an optical component according to one aspect of the disclosed subject matter. [Figure 20]

[0053] This is a flowchart showing a method for aligning optical features of an optical component according to one aspect of the disclosed subject matter. [Figure 21]

[0054] This figure shows various optical paths within an optical pulse stretcher according to one aspect of the disclosed subject matter. [Figure 22]

[0055] This figure shows various optical paths within an optical pulse stretcher according to one aspect of the disclosed subject matter. [Figure 23A]

[0056] This figure shows various optical paths within an optical pulse stretcher according to one aspect of the disclosed subject matter. [Figure 23B]

[0057] This figure shows various alignment conditions for light rays in an optical pulse stretcher according to one aspect of the disclosed subject matter. [Figure 24]

[0058] This figure shows various optical paths within an optical pulse stretcher according to one aspect of the disclosed subject matter. [Figure 25]

[0059] This figure shows various optical paths within an optical pulse stretcher according to one aspect of the disclosed subject matter. [Figure 26]

[0060] This is a flowchart showing a method for aligning optical features of an optical component according to one aspect of the disclosed subject matter. [Figure 27]

[0061] This figure shows various optical paths within an optical pulse stretcher according to one aspect of the disclosed subject matter. [Figure 28]

[0062] This figure shows various optical paths within an optical pulse stretcher according to one aspect of the disclosed subject matter. [Figure 29A]

[0063] This figure shows patterns that can be produced by various alignments of back-reflected rays in an optical pulse stretcher, according to one aspect of the disclosed subject matter. [Figure 29B]

[0063] This figure shows patterns that can be produced by various alignments of back-reflected rays in an optical pulse stretcher, according to one aspect of the subject matter disclosed. [Figure 29C]

[0063] This figure shows patterns that can be produced by various alignments of back-reflected rays in an optical pulse stretcher, according to one aspect of the subject matter disclosed. [Figure 30A]

[0064] This figure shows an embodiment of an aperture plate within a module for aligning back-reflected rays in an optical pulse stretcher, according to one aspect of the disclosed subject matter. [Figure 30B]

[0064] This figure shows an embodiment of an aperture plate in a module for aligning back-reflected rays in an optical pulse stretcher, according to one aspect of the subject matter disclosed. [Figure 30C]

[0064] This figure shows an embodiment of an aperture plate in a module for aligning back-reflected rays in an optical pulse stretcher, according to one aspect of the subject matter disclosed.

[0032]

[0065] Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, will be described in detail below with reference to the accompanying drawings. It should be noted that the present invention is not limited to the specific embodiments described herein. Such embodiments are described herein for illustrative purposes only. Those skilled in the art will readily come up with further embodiments based on the teachings contained herein. [Modes for carrying out the invention]

[0033]

[0066] This specification discloses one or more embodiments incorporating features of the present invention. The disclosed embodiments are merely illustrative of the present invention. The scope of the present invention is not limited to the disclosed embodiments. The present invention is defined by the claims appended herein.

[0034]

[0067] Where one or more embodiments are described herein, and where “one embodiment,” “a particular embodiment,” “exemplary embodiment,” etc. are used herein, it is understood that one or more embodiments described may include certain features, structures, or characteristics, but each embodiment may not necessarily include those features, structures, or characteristics. Furthermore, such phrases do not necessarily refer to the same embodiment. Moreover, where certain features, structures, or characteristics are described in relation to a particular embodiment, it is understood that performing such features, structures, or characteristics in relation to other embodiments, whether expressly described or not, is within the knowledge of those skilled in the art.

[0035]

[0068] Spatially relative terms such as "beneath," "below," "lower," "above," "on," and "upper" may be used herein to facilitate the description of the relationship between one element or feature and one or more other elements or features, as shown in the figures. Spatially relative terms are intended to encompass various orientations of the device in use or operation, in addition to the orientation shown in the figures. The device may be oriented in other ways (it may be rotated 90 degrees or in other directions), and the spatially relative descriptive terms used herein may be interpreted accordingly.

[0036]

[0069] Before describing specific embodiments in detail, it is beneficial to show exemplary environments in which embodiments of the present invention can be carried out. Referring to Figure 1, the photolithography system 100 includes an illumination system 105. As will be described more fully below, the illumination system 105 includes a light source that generates a pulsed light beam 110 and directs it to a photolithography exposure apparatus or scanner 115. The photolithography exposure apparatus or scanner 115 patterns microelectronic features on a wafer 120. The wafer 120 is placed on a wafer table 125. The wafer table 125 is constructed to hold the wafer 120 and is connected to a positioner (not shown) configured to precisely position the wafer 120 according to specific parameters.

[0037]

[0070] The photolithography system 100 uses a light beam 110 having a wavelength in the deep ultraviolet (DUV) range, such as 248 nanometers (nm) or 193 nm. The minimum size of microelectronic features that can be patterned on the wafer 120 depends on the wavelength of the light beam 110, with shorter wavelengths allowing for smaller minimum feature sizes. When the wavelength of the light beam 110 is 248 nm or 193 nm, the minimum size of microelectronic features may be, for example, 50 nm or less. The bandwidth of the light beam 110 may be the actual instantaneous bandwidth of the optical spectrum (or emission spectrum) containing information about how the light energy of the light beam 110 is distributed across various wavelengths. The scanner 115 includes an optical mechanism having, for example, one or more condenser lenses, a mask, and an objective system mechanism. The mask is movable along one or more directions, for example, along the optical axis of the light beam 110 or in a plane perpendicular to the optical axis. The objective system mechanism includes a projection lens that enables image transfer from the mask to the photoresist on the wafer 120. The illumination system 105 adjusts the angular range of the light beam 110 incident on the mask. The illumination system 105 also homogenizes (makes uniform) the intensity distribution of the light beam 110 in the mask.

[0038]

[0071] The scanner 115 may include, among several features, a lithography controller 130, an air conditioning device, and a power supply for various electrical components. The lithography controller 130 controls how layers are printed on the wafer 120. The lithography controller 130 includes a memory that stores information such as a process recipe. The process program or recipe determines the exposure length for the wafer 120, for example, based on the mask used and other factors that affect exposure. During lithography, multiple pulses of the light beam 110 illuminate the same area of ​​the wafer 120 to form an illumination dose.

[0039]

[0072] Furthermore, the photolithography system 100 preferably also includes a control system 135. Generally, the control system 135 includes one or more of the following: digital electronic circuits, computer hardware, firmware, and software. The control system 135 also includes memory, which may be read-only memory and / or random-access memory. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory, but examples include semiconductor memory devices such as EPROMs, EEPROMs, and flash memory devices, magnetic disks such as internal hard disks and removable disks, magneto-optical disks, and CD-ROM disks.

[0040]

[0073] The control system 135 may also include one or more input devices (such as a keyboard, touchscreen, microphone, mouse, or handheld input device) and one or more output devices (such as a speaker or monitor). Furthermore, the control system 135 may include one or more programmable processors and one or more computer program products tangibly embodied in a machine-readable storage device for execution by the one or more programmable processors. Each of the one or more programmable processors can execute a program of instructions and perform a desired function by performing actions on input data and generating appropriate outputs. Generally, the processors receive instructions and data from memory. Any of the above can be supplemented by or incorporated into a specially designed ASIC (Application-Specific Integrated Circuit). The control system 135 can be centralized or distributed partially or entirely throughout the photolithography system 100.

[0041]

[0074] Referring to Figure 2, an exemplary laser source system within the illumination system 105 is a pulsed laser source that generates a pulsed laser beam as an optical beam 110. Figure 2 illustrates a gas discharge laser system in an exemplary block diagram according to an embodiment of a particular aspect of the subject matter disclosed. The gas discharge laser system may include, for example, a solid or gas discharge seed laser system 140, an amplification stage such as a power ring amplifier ("PRA") stage 145, a relay optical system 150, and a laser system output subsystem 160. The seed system 140 may include, for example, a master oscillator ("MO") chamber 165.

[0042]

[0075] The seed laser system 140 may also include a master oscillator output coupler ("MOOC") 175. The MOOC 175 may include a partial reflection mirror and, together with a reflection grating (not shown) in a line narrowing module ("LNM") 170, form an oscillator cavity. Within this cavity, the seed laser 140 oscillates to form seed laser output pulses, i.e., the master oscillator ("MO"). The system may also include a line-center analysis module ("LAM") 180. The LAM 180 may include an etalon spectrometer for fine wavelength measurements and a coarse-resolution grating spectrometer. The MO wavefront engineering box ("WEB:wavefront engineering box") 185 functions to redirect the output of the MO seed laser system 140 to the amplification stage 145, which may include, for example, beam expansion by a multiprism beam expander (not shown) and coherence busting in the form of, for example, an optical delay path (not shown).

[0043]

[0076] The amplification stage 145 may include, for example, a PRA laser oscillation chamber 200. This can be an oscillator formed by a seed beam incidence and output coupling optical system (not shown), which may be incorporated into, for example, a PRA web 210, and the beam can be redirected backward through a gain medium within the chamber 200 by a beam inverter 220. The PRA web 210 may incorporate a partial reflection input / output coupler (not shown), a maximum reflection mirror for the nominal operating wavelength (e.g., about 193 nm in an ArF system), and one or more prisms.

[0044]

[0077] A bandwidth analysis module ("BAM") 230 at the output of the amplification stage 145 receives the output laser beam pulse from the amplification stage and can select a portion of the beam for metronome purposes, such as measuring the output bandwidth and pulse energy. The laser output beam pulse then passes through an optical pulse stretcher ("OPuS") 240 and a combined autoshutter metronome module ("CASMM") 250, which may be located at the position of the pulse energy meter. One purpose of the OPuS 240 is, for example, to convert a single output laser pulse into a pulse train. Secondary pulses generated from the initial single output pulse can be delayed relative to each other. By distributing the initial laser pulse energy within the secondary pulse train, the effective pulse length of the laser can be extended while simultaneously reducing the peak pulse intensity. Thus, the OPuS 240 can receive the laser beam from the PRA WEB 210 via the BAM 230 and guide the output of the OPuS 240 to the CASMM 250. Other appropriate configurations may be used in other embodiments.

[0045]

[0078] The PRA laser oscillation chambers 200 and MO165 are configured as chambers, within which a discharge between electrodes induces a laser oscillation gas discharge in the laser oscillation gas, generating a population inversion of high-energy molecules, such as Ar, Kr, and / or Xe, thereby producing radiation with a relatively broad bandwidth. This radiation can be line-narrowed to a relatively very narrow bandwidth and center wavelength selected in the line-narrowing module ("LNM") 170. This is known in the art.

[0046]

[0079] Typically, adjustments are made in LNMs. A common technique used for line narrowing and laser adjustment is to create a window at the rear of the laser discharge cavity through which a portion of the laser beam passes when it enters the LNM. Here, a portion of the beam is expanded by a prism beam expander and guided to a grating, which reflects a narrow selection of the laser's broad spectrum back into the discharge chamber, where it is amplified. Laser adjustment is typically performed by changing the angle at which the beam illuminates the grating, for example, using an actuator such as a piezo actuator.

[0047]

[0080] During operation, the OPuS240 extends an excimer laser or other gas discharge laser, such as a molecular fluorine gas discharge laser, which has a given pulse duration and TIS, to generate a long pulse with several peaks and an enlarged TIS.

[0048]

[0081] Figure 3 is a schematic front view of an example of an optical pulse stretcher 401 having a first optical pulse stretcher 401a and a second optical pulse stretcher 401b according to some embodiments of the present disclosure. The optical pulse stretcher 401 receives an input beam pulse 411, stretches it, and outputs a stretched output beam pulse 413.

[0049]

[0082] According to several embodiments and as will be discussed in more detail below, the second optical pulse stretcher 401b may include two or more (e.g., three) stages of a confocal optical pulse stretcher. In some examples, these three stages of the confocal optical pulse stretcher can be positioned substantially parallel to each other within the second optical pulse stretcher 401b. In some embodiments, the second optical pulse stretcher 401b may be positioned perpendicular or substantially perpendicular to the first optical pulse stretcher 401a. In other words, in some embodiments, the first optical pulse stretcher 401a (e.g., an orthogonal optical pulse stretcher that can be positioned vertically) is positioned perpendicular or substantially perpendicular to two or more (e.g., three) stages of the confocal optical pulse stretcher of the second optical pulse stretcher 401b, which is positioned vertically in the figure. According to some embodiments, the second optical pulse stretcher 401b is designed to provide additional optical delay.

[0050]

[0083] According to some embodiments, the extended optical pulse stretcher 401 combines two or more confocal optical pulse stretchers. For example, the extended optical pulse stretcher 401 combines confocal optical pulse stretchers in combinations of 4 reflections, 4 reflections, 12 reflections, and 12 reflections per optical circuit configuration. According to some embodiments, by including different combinations of mirror separation and delay path lengths (e.g., delay lengths for 4 reflections and 12 reflections), extremely long pulse stretching and minimal efficiency loss can be achieved as a result.

[0051]

[0084] According to some embodiments, the second optical pulse stretcher 401b may include three stages of a confocal optical pulse stretcher. However, embodiments of the present disclosure are not limited to these examples, and the second optical pulse stretcher 401b may include any other number of stages of a confocal optical pulse stretcher. In some examples, the first stage of the second optical pulse stretcher 401b is considered to have two mirrors. However, embodiments of the present disclosure are not limited to these examples, and the first stage of the second optical pulse stretcher 401b may include any other number (e.g., two or more) and / or configuration of mirrors. In some examples, the multiple mirrors used in the first stage of the second optical pulse stretcher 401b are configured to produce four reflections of the laser beam between them.

[0052]

[0085] In some examples, the second stage of the second optical pulse stretcher 401b is considered to have four mirrors. However, embodiments of the present disclosure are not limited to these examples, and the second stage of the second optical pulse stretcher 401b may include other numbers (e.g., four or more) and / or configurations of mirrors. In some examples, the multiple mirrors used in the second stage of the second optical pulse stretcher 401b are configured to produce 12 reflections of the laser beam between them.

[0053]

[0086] In some examples, the third stage of the second optical pulse stretcher 401b is considered to have four mirrors. However, embodiments of the present disclosure are not limited to these examples, and the third stage of the second optical pulse stretcher 401b may include other numbers (e.g., four or more) and / or configurations of mirrors. In some examples, the multiple mirrors used in the third stage of the second optical pulse stretcher 401b are configured to produce 12 reflections of the laser beam between them.

[0054]

[0087] According to some embodiments, the stages of the first optical pulse stretcher 401a and the second optical pulse stretcher 401b are designed such that the optical delay increases from the first optical pulse stretcher 401a to the second optical pulse stretcher 401b. Furthermore, the optical delay of each stage of the second optical pulse stretcher 401b increases from the first stage to the third stage. For example, the first optical pulse stretcher 401a (e.g., an orthogonal optical pulse stretcher) may have a certain optical delay. The first stage of the second optical pulse stretcher 401b may have a first optical delay greater than or equal to the optical delay of the first optical pulse stretcher 401a. The second stage of the second optical pulse stretcher 401b may have a second optical delay greater than or equal to the first optical delay. The third stage of the second optical pulse stretcher 401b may have a third optical delay greater than or equal to the second optical delay. According to some embodiments, the optical delay can be determined based on the distance the beam travels within the optical pulse stretcher.

[0055]

[0088] According to some embodiments, the first stage of the second optical pulse stretcher 401b may have an optical design including two mirrors (e.g., two lower mirrors 501 and 502 in Figure 3) that generate four reflections of the laser beam between them. While this example is considered using two mirrors, the first stage of the second optical pulse stretcher 401b may also include a different number of mirrors (e.g., two or more mirrors). These mirrors can be positioned to generate four reflections of the laser beam between them. In some embodiments, the two mirrors of the first stage of the second optical pulse stretcher 401b can be separated from each other by a physical distance of about 2m to 4m. For example, the physical distance can be about 2.5m to 3.5m. These distances are given for illustrative purposes only, and other distances may be used in other embodiments. In some examples, the first stage of the second optical pulse stretcher 401b can perform optical pulse stretching with an optical delay of, for example, about 60ns to 80ns. For example, the optical delay is about 65ns to 75ns. For example, the optical delay is approximately 70 ns to 75 ns. It should be noted that the exemplary physical distance between the two mirrors and the given exemplary optical delay are not limiting to embodiments of the present disclosure. The first stage of the second optical pulse stretcher 401b can be designed to achieve various other physical distances and / or various optical delays.

[0056]

[0089] According to some embodiments, the mirrors of the first stage of the second optical pulse stretcher 401b (e.g., the two lower mirrors, mirrors 501 and 502) may include rectangular concave mirrors. For example, two large rectangular concave mirrors can be used, but other shapes may be used in other embodiments. According to some embodiments, the reflective surface of the mirror can be made spherically concave, and the distance between the two mirrors of the first stage of the second optical pulse stretcher 401b (e.g., the surfaces of the two lower mirrors, mirrors 501 and 502) can be equal to (or approximately equal to) the radius of curvature of each of the two mirrors. For example, the mirrors can be designed and positioned based on a telecentric design. According to some embodiments, the concave mirrors can be designed using orthogonal tip tilt adjustment and Z-axis (e.g., beam propagation direction) adjustment.

[0057]

[0090] According to some embodiments, the first stage of the second optical pulse stretcher 401b may include additional optical elements. In one example, the first stage of the second optical pulse stretcher 401b may include a beam splitter used to split the laser beam and generate copies of the laser beam. The beam splitter of the first stage of the second optical pulse stretcher 401b may have a reflectance of, for example, about 45% to 65%. In some examples, the beam splitter may have a reflectance of about 50% to 60%. However, embodiments of the present disclosure are not limited to these examples, and various other values ​​of reflectance can be used. In some examples, the reflectance of the beam splitter may depend on and / or be calculated based on the reflectance of mirrors used in the first stage of the second optical pulse stretcher 401b.

[0058]

[0091] According to some embodiments, the second stage of the second optical pulse stretcher 401b may have an optical design including four mirrors (e.g., four central mirrors 501 and 502 in Figure 3) that generate 12 reflections of the laser beam between them. While this example is considered using four mirrors, the second stage of the second optical pulse stretcher 401b may also include a different number of mirrors (e.g., four or more mirrors). These mirrors can be positioned to generate 12 reflections of the laser beam between them. In some embodiments, the two pairs of mirrors in the second stage of the second optical pulse stretcher 401b may be separated from each other by a physical distance of about 2m to 4m. For example, the physical distance may be about 2.5m to 3.5m. These distances are given for illustrative purposes only, and other distances may be used in other embodiments. In some examples, the second stage of the second optical pulse stretcher 401b may perform optical pulse stretching with an optical delay of, for example, about 170ns to 210ns. For example, the optical delay is approximately 180 ns to 190 ns. For example, the optical delay is approximately 185 ns to 195 ns. It should be noted that the exemplary physical distance between the two pairs of mirrors and the exemplary optical delays given are not limiting to embodiments of the present disclosure. The second stage of the second optical pulse stretcher 401b can be designed to achieve various other physical distances and / or various optical delays.

[0059]

[0092] According to some embodiments, the mirrors of the second stage of the second optical pulse stretcher 401b (e.g., the four central mirrors of mirrors 501 and 502) may include rectangular concave mirrors. For example, four large rectangular concave mirrors can be used, but other shapes may be used in other embodiments. According to some embodiments, the reflective surface of the mirror can be made spherically concave, and the distance between the two pairs of mirrors of the second stage of the second optical pulse stretcher 401b (e.g., the surfaces of the two pairs of central mirrors of mirrors 501 and 502) can be equal to (or approximately equal to) the radius of curvature of each of the four mirrors. For example, the mirrors can be designed and positioned based on a telecentric design. According to some embodiments, the concave mirrors can be designed using orthogonal tip tilt adjustment.

[0060]

[0093] According to some embodiments, the second stage of the second optical pulse stretcher 401b may include additional optical elements. In one example, the second stage of the second optical pulse stretcher 401b may include a beam splitter (the central beam splitter of beam splitter 503 in Figure 3) used to split the laser beam and generate copies of the laser beam. The beam splitter of the second stage of the second optical pulse stretcher 401b may have a reflectance of, for example, about 45% to 65%. In some examples, the beam splitter may have a reflectance of about 50% to 60%. However, embodiments of the present disclosure are not limited to these examples, and various other values ​​of reflectance may be used. In some examples, the reflectance of the beam splitter may depend on and / or be calculated based on the reflectance of mirrors used in the second stage of the second optical pulse stretcher 401b.

[0061]

[0094] Further details regarding the optical pulse stretcher can be obtained from U.S. Patent No. 7,369,597, entitled “Laser Output Light Pulse Stretcher,” issued on May 6, 2008. The entire content of this patent is incorporated herein by reference.

[0062]

[0095] Figure 4 shows a schematic diagram of a portion of the laser beam path in a second optical pulse stretcher 401b according to some embodiments of the present disclosure.

[0063]

[0096] As shown in Figure 4, the laser beam 601, optically stretched using the stage of the first optical pulse stretcher 401a, is incident on the second optical pulse stretcher 401b. Using the first beam splitter 503a, the laser beam 601 is split into laser beam 603 and laser beam 605. Laser beam 605 is incident on the second stage of the second optical pulse stretcher 401b. Laser beam 603 is incident on the first stage of the second optical pulse stretcher 401b, which includes two mirrors. As shown in Figure 5A, after four reflections from the two mirrors 501a and 502a of the first stage of the second optical pulse stretcher 401b, a portion of the laser beam is reflected by the beam splitter 503a and incident on the second stage of the second optical pulse stretcher 401b, while the remainder of the beam continues to loop further inside the optical pulse stretcher 400.

[0064]

[0097] The laser beam 605 (and / or the laser beam from the first stage of the second optical pulse stretcher 401b) is split into laser beam 607 and laser beam 609. Laser beam 609 is incident on the third stage of the second optical pulse stretcher 401b. Laser beam 607 is incident on the second stage of the second optical pulse stretcher 401b, which includes four mirrors 501c, 501b, 502b, and 501c as shown in Figure 5B. After 12 reflections from the four mirrors identified by numbers 1 to 12 in the second stage of the second optical pulse stretcher 401b, a portion of the laser beam is reflected by the beam splitter 503b and incident on the third stage of the second optical pulse stretcher 401b.

[0065]

[0098] The laser beam 609 (and / or the laser beam from the second stage of the second optical pulse stretcher 401b) is split into laser beam 611 and laser beam 613. Laser beam 613 is reflected back to the first optical pulse stretcher 401a using mirrors 505a and 505b. Laser beam 611 is incident on the third stage of the second optical pulse stretcher 401b, which includes four mirrors. After twelve reflections from the four mirrors of the third stage of the second optical pulse stretcher 401b, a portion of the laser beam is reflected back to the first optical pulse stretcher 401a using beam splitter 503c and folding mirrors 505a and 505b (Figure 4).

[0066]

[0099] Figure 6 shows a schematic diagram of a portion of the laser beam path within a second optical pulse stretcher 401b and a portion of the mirrors used in the second optical pulse stretcher 401b, according to some embodiments of the present disclosure.

[0067]

[0100] Figure 6 illustrates five mirrors on one side of the second optical pulse stretcher 401b. According to some embodiments, it will be understood that there are also substantially symmetrical configurations within the second optical pulse stretcher 401b. In this example, the mirror 502a of the first stage of the second optical pulse stretcher 401b is shown. A mirror (e.g., mirror 501a) exists on the other side of the first stage of the second optical pulse stretcher 401b, but is not shown in this figure. In this example, a pair of mirrors 502b and 502c of the second stage of the second optical pulse stretcher 401b is illustrated. Another pair of mirrors (e.g., a pair of mirrors 501b and 501c) exists on the other side of the second stage of the second optical pulse stretcher 401b, but is not shown in this figure. Also in this example, a pair of mirrors 502d and 502e of the third stage of the second optical pulse stretcher 401b is illustrated. Another pair of mirrors (for example, a pair of mirrors 501d and 501e) exists on the other side of the third stage of the second optical pulse stretcher 401b, but is not shown in this figure.

[0068]

[0101] The following discussion will focus on a configuration in which optical components within the OPuS, such as mirrors, are arranged in two banks that are substantially symmetrical with respect to the central axis, in order to provide concrete examples that facilitate the explanation. However, it should be acknowledged that the specific examples described herein are not limiting, as the principles described herein are applicable to other configurations as well. As used herein, “substantially symmetrical” and “substantially symmetrical” mean that the OPuS is sufficiently symmetrical so that it can function to achieve its intended purpose and that the image integration module described below can “see” all of the mirrors simultaneously. In one embodiment, the image integration module in such a configuration is configured to collect object rays from both the left and right optical components, which in this example are concave mirrors. Thus, in this configuration, there are several pairs of left and right mirrors. A camera system, including a camera and lens system, is positioned outside the sealed OPuS casing. The camera is configured to collect rays through a sealed window that is transparent to the visible portion of the spectrum, i.e., light in the wavelength range of approximately 380 to approximately 700 nm. These rays form an image. Half of this image arises from the left concave mirror of the mirror pair, and the other half arises from the right concave mirror of the mirror pair.

[0069]

[0102] Against this backdrop, a conventional method for aligning optical elements within an OPuS is described in relation to Figure 7. As shown in Figure 7, the OPuS 700 includes an outer casing 710. Optical elements 720, 730, 740, and 750 are positioned within the outer casing 710. These optical elements can be, for example, mirrors. These optical elements must be aligned so that the incoming beam strikes the correct position on each optical element. To perform this alignment procedure, the outer casing 710 is opened, and an alignment card 760 is placed adjacent to the optical surface of one of the optical elements, in this figure, optical element 750. The optical elements are then aligned so that the beam strikes the correct position on the alignment card 760. As mentioned above, this method has several drawbacks. These drawbacks include, for example, the need to open and purge the outer casing 710, and the possibility of exposing the operator's hands to ultraviolet radiation because the operator needs to insert their hands into the outer casing 710 in an open beam situation. Furthermore, this increases the risk of optical contamination, resulting in a shorter lifespan for optical components.

[0070]

[0103] In one embodiment, as shown in Figure 8A, the OPuS800, which enables an improved alignment method, includes an enclosure 810. Located within the enclosure 810 are optical features 820, 830, 840, and 850. Located in the center of the enclosure 810 is an image integration module 860. As will be described in more detail below, this image integration module 860 collects light from the optical features and provides this light to the camera 890 via a sealed window 870 and lens 880. Essentially, the camera 890 "sees" all the optical features within the enclosure 810 simultaneously without needing to open the enclosure 810. This allows for continuous observation of the alignment state of the OPuS800 while avoiding the drawbacks of conventional methods. In some embodiments, the image integration module 860 is positioned as close to the camera system as possible to maximize the available field of view. As will be described in more detail below, elements 835 and 855 are adjusters. Figure 8B shows the placement of the image integration module 860 relative to the configuration shown in Figure 5B. In this case as well, numbers 1 through 12 indicate the positions of the 12 reflections from the four mirrors.

[0071]

[0104] Figure 9A shows a possible embodiment of the image integration module 860 according to an aspect of one embodiment. As shown, the image integration module 860 can be implemented as a pair of specular prisms 910, 920. Prism 910 receives light from a region of interest 825 including at least one optical feature 820 and is positioned to redirect this light outward through a sealed window 870 as shown. Similarly, prism 920 receives light from a region of interest 835 and is positioned to redirect this light through the sealed window 870 as shown. Thus, a camera system including a camera and one or more lenses positioned on the other side of the sealed window 870 receives image-forming light simultaneously from both the left region of interest 825 and the right region of interest 835. In other words, the image integration module 860 collects object rays from both the left optical feature and the right optical feature. The camera and lens system positioned outside the sealed OPuS casing 820 camera collects object rays through a sealed window transparent to visible light and generates an image. Half of this image comes from the optical feature on the left, and the other half comes from the optical feature on the right.

[0072]

[0105] Figure 9B shows how the image integration module 860 receives light from both the left region of interest 825, which contains at least one optical feature, and the right region of interest 835, which also contains at least one optical feature, and redirects this light towards position A. From position A, the virtual object 825a, which is the image from the left region of interest 825, and the virtual object 835a, which is the image from the right region of interest 835, appear to be juxtaposed, that is, positioned side by side, so that they can be viewed simultaneously by a single camera system located at position A.

[0073]

[0106] According to one embodiment, the image integration module 860 can also be implemented using a pair of mirrors. As an example, a mirror 950 is shown in Figure 10, and the other mirror is positioned substantially symmetrically with respect to axis 970. The conditions for the maximum field of view in such a configuration are established geometrically, as shown in Figure 10. In this figure, h is the height of the object's field of view 960, d is the vertical distance between the center of the field of view and the center of the mirror 950, and s is the horizontal distance between the field of view and the mirror 950. The upper part of the figure is the position of the virtual image as seen from below the mirror (in this figure). The angle θs for the maximum field of view between the object's field of view 960 and a line 980 that is parallel to the center of the mirror 950 is given by the following relationship.

[0074]

number

[0075] Therefore, the angle θs, which is the inclination angle of the mirror 950 with respect to the optical perpendicular, is given by the following:

[0076]

number

[0077]

[0107] As shown in the figures, the image integration module 860 can be implemented in any one of several ways. According to some embodiments, the image integration module 860 can be implemented as a pair of prisms 910, 920 as shown in Figure 9, or as a single prism 1000 having two mirror surfaces 1010 and 1020 as shown in Figure 11A, or as two flat, chamfered mirrors 1030 and 1040 as shown in Figure 11B. However, in some embodiments in which the image integration module 860 is implemented, gaps between optical elements or mirror surfaces are minimized, as shown in Figure 11C, so as not to waste any portion of the field of view lost in the gaps. In Figure 11C, arrow A indicates the field of view from one of the optical features, such as a mirror, arrow B indicates the field of view from the other optical feature in the pair, and arrow C indicates the field of view lost due to the gap between prisms 910, 920.

[0078]

[0108] Figure 9A shows a configuration in which light passing through the sealed window 870 travels straight and directly reaches the lens system 880. In some embodiments, it may be advantageous to interpose additional optical elements in the path from the sealed window 870 to the lens system 880. For example, Figure 12A shows a configuration in which a folding mirror 1100 is placed in this path to bend the optical path and provide a more compact configuration. In the configuration of Figure 12A, the mirror 1100 is positioned inside the casing to obtain a larger field of view. This configuration also provides the function of adjusting the orientation of the image and improving the size and shape matching between the field of view and the image sensor in the camera. Figure 12B shows a configuration in which prisms 910a and 920a are partially rotated to bend the optical path. Figure 12B shows how the image integration module 860a receives light from both the left region of interest 825 and the right region of interest 835 and redirects this light to position B. From position B, the virtual object 825b, which is the image of the left region of interest 825, and the virtual object 835b, which is the image of the right region of interest 835, appear to be juxtaposed, that is, positioned side by side, so that they can be viewed simultaneously by a single camera system located at position B. As shown in the figure, this configuration also provides the function of adjusting the orientation of the image and improving the size and shape correspondence between the field of view and the image sensor in the camera.

[0079]

[0109] In some embodiments, it may be advantageous to increase the visibility of the region of interest by providing alignment features and using visible light (to a camera) from fluorescence generated by the absorption of ultraviolet light. Using a dichroic mirror as an example of the optical element to be aligned, the mirror is generally supported by a mirror support plate 1300 including a support 1310 and at least one alignment feature 1320. According to some embodiments, as shown in Figure 13, the dichroic mirror assembly 1330 includes a substrate 1340 that transmits visible light and a UV reflective coating 1360 (Figure 13D), as shown in Figure 13B. The dichroic mirror assembly 1330 is superimposed on the mirror support plate 1300 to create a sandwich structure as shown in Figures 13C and 13D. The UV radiation striking the dichroic mirror assembly 1330 generates a visible-beam fluorescence footprint 1350, as will be further described below. The fluorescence and rear alignment features generated by the ultraviolet radiation can be observed, for example, by a camera operating within the visible spectral range. By comparing the position of the fluorescence with the alignment feature via a mirror or substrate, the system can be easily aligned as described below. While visible light is used in this example, it should be understood that radiation outside the visible region of the spectrum may also be used.

[0080]

[0110] According to various embodiments, the illumination beam footprint can be generated in one of several different ways upon exposure to UV. For example, as shown in Figure 14A, the UV reflective coating 1360 can be selected to exhibit inherent fluorescence properties upon exposure to UV. This is shown in Figures 14A to 14D, where thick arrows indicate incident UV radiation and wavy arrows indicate light generated by fluorescence. Alternatively, as shown in Figure 14B, a fluorescent coating 1370 can be provided on the rear surface of the substrate 1340, and light is generated by UV leakage through the reflective coating 1360. The term "rear surface" when referring to the substrate 1340 means the substrate surface facing away from the incoming UV radiation. Alternatively, as shown in Figure 14C, a fluorescent coating 1380 can be provided on the front surface of the support 1310, and light is generated by UV leakage through the reflective coating 1360. The term "front surface" when referring to the support 1310 means the support surface facing in the direction of the incoming UV radiation. Alternatively, as shown in Figure 14D, the support 1310 can be fabricated from a fluorescent material, and light is generated by UV leakage through the reflective coating 1360.

[0081]

[0111] Figure 15A is a plan view of the dichroic mirror assembly 1330 superimposed on a mirror support plate 1300 having alignment features 1320. The mirror support plate 1300 with alignment features 1320 is behind the dichroic mirror assembly 1330 and is therefore shown in perspective. UV radiation striking the dichroic mirror assembly 1330 generates a visible beam fluorescence footprint 1350 as described. Figure 15A shows a misaligned position where the fluorescence footprint 1350 does not adequately coincide with the alignment features 1320. Figure 15B shows a aligned position where the fluorescence footprint 1350 adequately coincides with the alignment features 1320. This is achieved by aligning the UV beam so that it is incident on the correct positions (two in this example) on the dichroic mirror assembly 1330. The fluorescence illuminates and reveals the relative positioning of the UV footprint and the alignment features. To correct the alignment, the orientation of one or more mirrors is adjusted so that the beam is incident at the correct position on all of these mirrors. Images of the beam incident adjacent to the alignment feature are captured by a camera, i.e., converted into digital images, which the operator can view while performing the alignment operation.

[0082]

[0112] According to some embodiments, the alignment of the pulse stretcher requires that at least some of the mirrors be adjustable; for example, in a four-mirror configuration, at least two of the four imaging relay mirrors must be adjustable. Since each of the two adjustable mirrors has tip / tilt adjustment, a total of four degrees of freedom are obtained. For a confocal design of the system, the two adjustable mirrors can be positioned at both ends of the OPuS. Also, according to some embodiments, the adjustable mirrors can be designed with Z-axis (e.g., beam propagation direction) adjustment.

[0083]

[0113] Typically, the adjustment of these components for alignment is performed using through-the-wall adjusters ("TWAs") such as adjusters 855 and 835 (Figure 8A). These involve the use of a manually operated hex driver to tip, tilt, or translate the optical components or modules. The TWA provides a sealed mechanical feedthrough to a particular adjuster, which is accessed, for example, through a cover via the sealed mechanical feedthrough. Adjustments can also be performed using electrically operated TWAs instead of manually operated ones. A motor is mechanically coupled to the TWA. For example, the motor has a shaft to which a hex adapter is attached so that when the motor rotates the shaft, the hex driver also rotates, allowing the end of the TWA to be translated along the axis in accordance with the direction of rotation of the shaft. By using an electrically operated TWA, the alignment process can be automated. The digital image from camera 890 is sent to control system 135 (Figure 1), which then analyzes the image and activates the TWA to perform the alignment.

[0084]

[0114] It should be understood that alignment may involve adjusting only one optical feature, and that adjusting a second optical feature may be necessary to direct the beam to the correct portion of the first optical feature.

[0085]

[0115] Figure 16 is a flowchart illustrating a process for aligning optical features positioned within a sealed enclosure, according to an embodiment. In step S10, the optical features within the enclosure are exposed to a UV emission beam. The UV emission beam visualizes its footprint. In step S20, the light generated by the UV emission beam fluorescence is combined to form a single image from the optical features. In step S30, the single image is sent to a camera outside the enclosed chamber. In step S40, the features are aligned based on the image generated by the combined light from the features, either by an engineer viewing the image captured by the camera or by a control system as described above. Essentially, in examples where each feature includes an alignment feature, the alignment is determined based on the beam footprint of each optical feature and the positional relationship of the alignment feature in the image.

[0086]

[0116] As shown in Figure 17, the confocal 4xOpuS1710 may include a beam splitter 1720 for coupling light in and out of the confocal OpuS1710. The confocal OpuS1710 also includes confocal concave mirrors CM1 and CM2, each having a focal length f and separated by a distance d approximately equal to twice the focal length f. Aligning the beam splitter 1710 is difficult because it requires multiple steps and depends on the alignment of the other mirrors.

[0087]

[0117] In the example of the 4xOpuS1710 shown in Figure 17, the position of the first beam injection spot on CM1, SP1 is controlled by the orientation of the beam splitter 1720. The position of the first beam injection spot on CM2, SP2 is controlled by the orientation of CM1, typically given by adjustable tip and tilt, and is also affected by the orientation of the beam splitter 1720. The position of the second beam injection spot on CM1, SP3 is controlled by the orientation of CM2, the orientation of the beam splitter 1720, and the orientation of CM1. Here and elsewhere, the term “orientation” refers at least to the orientation of the tip and tilt. However, the position of the second beam injection spot on CM2, SP4 is controlled almost entirely by the orientation of the beam splitter 1720. According to one embodiment, the beam splitter 1720 is arranged asymmetrically in the lateral direction, and the distance between SP4 and the beam splitter 1720 is about six times the distance between the beam splitter 1720 and SP1.

[0088]

[0118] As shown in Figure 18, the confocal 12xOpuS1810 may include a beam splitter 1820 for coupling light in and out of the confocal OpuS1810. The confocal OpuS1810 also includes a first pair of confocal concave mirrors CM1 and CM2, each having a focal length f and separated by a distance d approximately equal to twice the focal length f. The confocal OpuS1810 also includes a second pair of confocal concave mirrors CM3 and CM4, each having a focal length f and separated by a distance d approximately equal to twice the focal length f.

[0089]

[0119] In the example of the 12xOpuS1810 shown in Figure 18, the position of the first beam injection spot on CM1, SP1 is controlled by the orientation of the beam splitter 1820. The position of the first beam injection spot on CM4, SP2 is controlled by the orientation of CM1, which is typically given by the adjustable tip and tilt, and is also affected by the orientation of the beam splitter 1820. The position of the second beam injection spot on CM1, SP3 is controlled by the orientation of CM4, the orientation of the beam splitter 1820, and the orientation of CM1. However, the position of the first beam injection spot on CM4, SP4 is controlled almost entirely by the orientation of the beam splitter 1820.

[0090]

[0120] These pulse stretchers can be arranged in series across multiple stages. For example, the overall OpuS design may include one 4xOpuS alignment block as shown in Figure 17 and two 12xOpuS alignment blocks as shown in Figure 18.

[0091]

[0121] Currently, for example, aligning a beam splitter in a 4xOpuS alignment block involves several alignment steps. These steps must be performed so that the beam reaches SP4 and that SP1 and SP4 are aligned together by beam splitter adjustment. If the beam deviates from one of the concave mirrors during beam splitter alignment, an additional realignment step is required.

[0092]

[0122] Figure 19 is a flowchart illustrating a method for aligning a beam splitter in a 4xOpuS alignment block without the subjective advantages disclosed herein. In step S100, the beam splitter is aligned so that the beam injection spot SP1 is aligned to the correct beam position on the first concave mirror CM1. In step S110, the concave mirror CM1 is aligned so that the injection spot SP2 is aligned to the correct beam position on the second concave mirror CM2. In step S120, CM2 is aligned so that the third injection spot SP3 is aligned to the correct beam position on the first concave mirror CM1. In step S130, the beam splitter is realigned so that SP1 and SP4 are aligned to the correct beam positions. In step S140, it is determined whether the beam has deviated from CM1 or CM2 during the alignment process. If the beam has deviated from one of the concave mirrors during the alignment process, the process must return to step S110 and be restarted. If the beam does not deviate from one of the concave mirrors, the process ends in step S150.

[0093]

[0123] Figure 20 is a flowchart illustrating a method for aligning a beam splitter in a 12xOpuS alignment block without the subjective advantages disclosed herein. In step S200, the beam splitter is aligned so that the beam injection spot SP1 is aligned to the correct beam position on the first concave mirror CM1. In step S210, the concave mirror CM1 is aligned so that the injection spot SP2 is aligned to the correct beam position on the concave mirror CM4. In step S220, CM4 is aligned so that the third injection spot SP3 is aligned to the correct beam position on the first concave mirror CM1. In step S230, the beam splitter is realigned so that SP1 and SP4 are aligned to the correct beam positions. In step S240, it is determined whether the beam has deviated from CM1 or CM4 during the alignment process. If the beam has deviated from one of these concave mirrors during the alignment process, the process must return to step S210 and be repeated. If the beam does not deviate from one of the concave mirrors, the process terminates in step S250.

[0094]

[0124] In the configuration described above, where OpuS consists of multiple blocks, the overall alignment process involves many steps, and may involve even more steps if the beam "goes off" the mirror, i.e., if it is sent out in a way that avoids collision with the mirror. Consequently, the time required to perform the entire alignment process can be long. This leads to the undesirable consequence of a longer mean time to repair (MTTR) during laser installation and servicing (approximately 30 to 60 minutes). Reducing MTTR is key to improving system availability and lowering service costs.

[0095]

[0125] One way to shorten the MTTR is to take advantage of the fact that in a well-aligned confocal OpuS, there is virtually perfect collinearity between the directly passing beam and the split beam after passing through the OpuS. Referring to Figure 21, the incoming ray 2100, i.e., the ray that travels upward in the figure, enters block 2110, and hits beam splitter 2120, becomes the "outgoing ray" 2130, i.e., the directly passing ray 2130 that travels upward in Figure 21. The rest of the incoming ray 2100 is deflected by beam splitter 2120 as ray 2150 to SP1. This ray 2150 travels through a closed path within OpuS 2110. One segment of this path is ray 2140 traveling from SP4 to beam splitter 2120, which is collinear with ray 2150. Ray 2140 is deflected upward as ray 2160 by beam splitter 2120, and ray 2160 coincides with beam 2130 in a well-aligned OpuS. This is also shown in Figures 22 and 23.

[0096]

[0126] Since ray 2130 is collinear with ray 2160, when ray 2130 is back-reflected, a portion of ray 2130 that is split by beam splitter 2120 and guided to SP4, namely ray 2170 in Figure 23A, becomes collinear with ray 2140, and both beams overlap on SP4. Therefore, SP4 can be re-established using a simple back-reflected beam of ray 2130 without aligning CM1 and CM2. This has the potential to significantly simplify the beam splitter alignment procedure.

[0097]

[0127] This is also shown in Figure 23A. Here, the incoming ray 2100 is split by the beam splitter 1720, and a portion of the ray 2130 hits the retroreflector 2200. As is known, a retroreflector (sometimes called a retroflector or cataphote) is a device or surface that reflects radiation back to its source. The retroreflector 2200 reflects the ray 2130 as ray 2135. Ray 2135 hits the beam splitter 2100 and is reflected towards SP4 as ray 2170. At the same time, the ray 2140 from SP4 to the beam splitter 1720 travels along the same path between SP4 and the beam splitter 1720, so rays 2140 and 2170 are collinear. This means that the footprints of these rays coincide and overlap at SP4. A diagram of this overlap condition is shown in Figure 23B. Here, rays 2170 and 2140 strike the portion of the concave mirror CM2 having spot SP4. As described above, the beam incidence spot can be visualized by providing a material that fluoresces under ultraviolet light in the portion of CM2 where spot SP4 is generated. Generally, the incidence spots of rays 2140 and 2170 are precisely mapped to each other, but it should be noted that these incidence spots may not be perfectly circular as shown in the figure, for example, if ray 2140 deviates from CM1 and CM2, ray 2140 may be cut off by CM1 or CM2. However, when the beam splitter 1720 is adjusted, the incidence spots of both beams generally move synchronously together. The dashed circle 2180 indicates a preferred beam position where rays 2140 and 2170 are guided together by adjusting the beam splitter 1720 (Figure 23A).

[0098]

[0128] Figure 24 shows this configuration for a 12xOpuS block 2300. Here, the incoming ray 2310 is split by the beam splitter 2320, with part of the ray 2130 passing through the beam splitter 2320 and the ray 2330 hitting the retroreflector 2340. The retroreflector 2340 reflects the ray 2330 into ray 2350. The ray 2350 hits the beam splitter 2320 and is reflected towards spot SP4 as ray 2360. At the same time, in a well-aligned 12XOPuS, there is a perfect collinear relationship between the directly passing beam 2360 and the beam 2370 passing through the OpuS. In other words, the ray 2370 from SP4 to the beam splitter 2320 travels along the same path between SP4 and the beam splitter 2320, and for this reason rays 2360 and 2370 are collinear. This means that these rays virtually perfectly coincide and overlap in SP4.

[0099]

[0129] The above explanation assumes that beam splitter 2120 has no thickness, but the above conclusions are valid even if beam splitter 2120 has a non-zero thickness. Furthermore, since SP4 can be easily reproduced by the back-reflected beam, the above conclusions also apply to 12xOpuS alignment. For example, by using the visible SP1 and SP4 obtained by the camera method described above, the beam splitter can be adjusted based only on these two beam positions. This provides a rapidly performable and significantly simplified OpuS beam splitter alignment procedure.

[0100]

[0130] As shown in Figure 25, the above configuration provides special advantages in a multi-stage optical pulse stretcher. In Figure 25, the first-stage optical pulse stretcher 2500 includes two confocal mirrors 2510 and 2520 and receives the input ray 2530. The input ray 2530 is split by a beam splitter 2540 into a beam that travels toward SP1, and the other portion is split and travels upward in the figure to a second beep splitter 2550 of the second stage 2560 of the optical pulse stretcher. This stage 2560 includes a first mirror 2570 and a second mirror 2580, which continues to the nth stage 2590 of the optical pulse stretcher, which has a beam splitter 2600 and mirrors 2610 and 2620, as indicated by dots. Above this nth stage is a retroreflector 2630 that reflects the beam traveling upward in the figure. The reflected light rays strike these beam splitters and are successively divided, with each divided ray generating a spot corresponding to SP4. Therefore, all stages can be aligned using a single input ray and a single retroreflector.

[0101]

[0131] For example, Figure 26 shows a simplified procedure for aligning beam splitters in a multi-stage OPuS. In step S300, where this procedure begins, the retroreflector is aligned to reflect the "ray out" beam, i.e., the ray passing through the beam splitter. Next, in step S310, the beam splitter is aligned based on the positions of SP1 and SP4 in the first stage OpuS. Then, in step S320, the beam splitter in the second stage of the OPuS is aligned based on spots SP1 and SP4 in the second stage OPuS. This continues until step S330, which relates to the final stage of the OpuS. In step S330, beam splitter N is aligned based on the positions of SP1 and SP4 in the N-stage OPuS. At this point, the procedure ends in step S340.

[0102]

[0132] Figure 27 shows a configuration similar to that of Figure 25, but differs in that the retroreflector is implemented as a turning mirror or a folding mirror 2700 and reflector 2710. This configuration allows for bending of the optical path and enables a more compact configuration for the retroreflector.

[0103]

[0133] Proper adjustment of the retroreflector can be achieved by observing the position of the retroreflected beam after it has passed the first stage of the OpuS. Figure 28 shows a configuration that achieves this. In Figure 28, the retroreflected beam 2800 enters a module 2810 which includes a plate 2820 having an aperture 2830. When the retroreflector is properly adjusted, the ray 2800 illuminates the edge of the aperture 2830. The plate 2820 is reflective to ultraviolet radiation due to the material it is made from or because it has a reflective coating 2840 as shown. The reflection from the periphery of the aperture 2830 glows on the fluorescent screen 2850. The alignment of the retroreflector can be assessed according to the characteristics of the image on the fluorescent screen 2850.

[0104]

[0134] Figures 29A, 29B, and 29C illustrate various possibilities for the image on the fluorescent screen. Figure 29A shows a cross-section of an aperture image where the profile symmetrically follows the edge of the aperture, suggesting that the retroreflector is properly calibrated. It should be noted that the image of a circular aperture is slightly elliptical because the plate is positioned at a certain angle to the back-reflected beam. Figure 29B shows a cross-section of an aperture image where the profile is off-center relative to the aperture, suggesting that the retroreflector is not properly oriented. Figure 29C is another example of a cross-section of an aperture image where the profile does not symmetrically follow the edge of the aperture, suggesting that the retroreflector is not properly calibrated.

[0105]

[0135] Figures 30A, 30B, and 30C show various details of an aperture plate used according to a specific aspect of one embodiment. Figure 30A is a side cut of the aperture plate 2820, where the aperture 2830 is positioned to receive the back-reflected beam 2800, ideally reflecting a portion of the beam 2860 from the entire edge of the aperture 2830. As described above, a reflective coating 2840 can be placed on the side of the plate 2820 into which the beam 2800 is incident. To avoid obstruction of the beam passing through the aperture when the plate is tilted, the aperture has a slanted profile. These details can also be seen in Figures 30B and 30C.

[0106]

[0136] In the diagram, the diameter of the downward-moving back-reflected beam is slightly larger than the aperture due to the finite exitivity of the laser beam, and the reflected beam exhibits a similar profile to the aperture. During the alignment of the back-reflected beam, the entire module 2810 (Figure 28) can be inserted into the beam and removed from the beam once alignment is complete. Plate 2820 can be made from a material such as aluminum, which has a high intrinsic UV reflectivity when polished by, for example, diamond turning. Alternatively, plate 2820 can be made from glass / ceramic material using a surface coating with a UV-reflective coating.

[0107]

[0137] The present invention is made with the help of functional building blocks that describe implementation forms of specific functions and their relationships. The boundaries of these functional building blocks are arbitrarily defined herein for the sake of clarity. Alternative boundaries may be defined as long as the specific functions and their relationships are adequately implemented.

[0108]

[0138] The above-mentioned descriptions of specific embodiments have sufficiently revealed the overall nature of the invention, and by applying knowledge of the art, such specific embodiments can be readily modified and / or adapted to various uses without excessive experimentation and without departing from the overall concept of the invention. Accordingly, such adaptations and modifications shall fall within the meaning and scope of equivalents of the disclosed embodiments based on the teachings and guidance presented herein. Since the expressions or terms herein are for illustrative purposes only and not limitation, it will be understood that the expressions or terms herein should be interpreted in terms of teachings and guidance by those skilled in the art.

[0109]

[0139] Further embodiments can be described using the following clauses. 1. A confocal optical pulse stretcher, The first concave mirror, A first concave mirror and a second concave mirror that is confocal, Retro reflector, A beam splitter optically positioned between a first concave mirror and a second concave mirror, further comprising a beam splitter arranged to divide a first beam traveling in a first direction into a first beam portion guided to the first concave mirror and a second beam portion traveling in the first direction, The first beam portion passes through a closed optical path in a confocal optical pulse stretcher, the optical path includes a segment between a second concave mirror and a beam splitter, and the retroreflector is positioned to backreflect the second beam portion and generate a backreflected beam that travels in a second direction opposite to the first direction. A beam splitter is further positioned to split the back-reflected beam into a third beam portion that passes through a segment and is incident on a second concave mirror, forming a confocal optical pulse stretcher. 2. The retroreflector includes a mirror with an adjustable tip and tilt, as described in Clause 1, and is a confocal optical pulse stretcher. 3. The confocal optical pulse stretcher according to Clause 1, comprising a folding mirror and a fixed mirror having adjustable tips and tilt, wherein the folding mirror is positioned to receive a second beam portion and redirect it toward the fixed mirror, and the fixed mirror is configured to reflect the second beam portion toward the folding mirror. 4. A reflective element having an aperture that receives back-reflected light that has passed through a beam splitter and travels in a second direction opposite to the first direction, and is positioned to redirect the light, A fluorescent screen positioned to receive the redirected light, A confocal optical pulse stretcher as described in Clause 1, further comprising the features described in Clause 1. 5. A confocal optical pulse stretcher as described in Clause 4, wherein the reflective element is oriented at a certain angle to the light passing through the beam splitter and has an aperture. 6. The confocal optical pulse stretcher according to Clause 4, wherein the UV reflective element has a reflective surface arranged to redirect light. 7. A device for facilitating the alignment of a beam splitter in a confocal optical pulse stretcher, The confocal optical pulse stretcher includes at least a first concave mirror and a second concave mirror that is confocal with the first concave mirror. The confocal optical pulse stretcher further includes a retroreflector. The beam splitter is optically positioned between the first concave mirror and the second concave mirror. The beam splitter is further positioned to divide the first beam into a first beam portion which is guided to the first concave mirror and a second beam portion which proceeds in the first direction toward the retroreflector. The retroreflector is positioned to back-reflect the second beam portion and generate a back-reflected beam. The first beam portion passes through the optical path within the confocal optical pulse stretcher, The beam path includes a segment between the second concave mirror and the beam splitter. The beam splitter is further positioned to divide the back-reflected beam into a first portion that proceeds to the second mirror and a second portion that passes through the beam splitter. The device is A reflecting element having an aperture that receives a second portion of the back-reflected beam light passing through the beam splitter and is positioned to redirect the light, A fluorescent screen positioned to receive the redirected light, A device equipped with the following features. 8. The apparatus according to Clause 7, wherein the reflecting element is oriented at an angle to a second portion of the back-reflected beam passing through the beam splitter and has an aperture. 9. The apparatus according to Clause 7, wherein the reflective element has a reflective surface positioned to redirect a second portion of the back-reflected beam. 10. A method for aligning a beam splitter in a confocal optical pulse stretcher, The confocal optical pulse stretcher includes at least a first concave mirror and a second concave mirror that is confocal with the first concave mirror. The beam splitter is optically positioned between a first spot on a first concave mirror and a second spot on a second concave mirror. The beam splitter is further positioned to divide the first incoming beam, which is traveling in the first direction, into a first beam portion that is guided to the first spot and a second beam portion that continues to travel in the first direction toward the retroreflector. The first beam portion passes through an optical path in a confocal optical pulse stretcher, the optical path includes a segment between the second spot and the beam splitter, and the first beam portion strikes the second spot on the second mirror. The method is, The second beam portion is reflected back to the beam splitter along a return path in the second direction opposite to the first direction, The second beam portion that has been reflected back is divided into a beam that travels from the beam splitter to the second spot and a third beam portion that continues to travel in the second direction, Align the split, back-reflected second beam portion so that it is incident on the second spot on the second mirror, Methods that include... 11. The method according to clause 10, further comprising illuminating a fluorescent screen with a portion of a third beam to generate an image, and aligning a retroreflector at least partially based on a determination of whether or not the image has predetermined characteristics. 12. The method according to Clause 11, wherein generating an image by directing a portion of the third beam onto a fluorescent screen includes reflecting at least a portion of the third beam onto a fluorescent screen using a reflective element having an aperture. 13. The method according to Clause 11, wherein the determination of whether an image has a predetermined feature includes determining whether the image includes light reflected symmetrically from the periphery of the aperture.

[0110]

[0140] The aforementioned embodiments and other embodiments are within the scope of the following claims.

Claims

1. A confocal optical pulse stretcher, The first concave mirror, The first concave mirror and the second concave mirror which is confocal, Retro reflector, A beam splitter optically positioned between the first concave mirror and the second concave mirror, further positioned to split a first beam traveling in a first direction into a first beam portion guided to the first concave mirror and a second beam portion traveling in the first direction, A reflecting element having an aperture that receives back-reflected light that has passed through the beam splitter and is traveling in a second direction opposite to the first direction, and that is arranged to redirect the back-reflected light, The system comprises a fluorescent screen arranged to receive the light whose direction has been changed, The first beam portion passes through the closed optical path in the confocal optical pulse stretcher, The closed optical path includes a segment between the second concave mirror and the beam splitter. The retroreflector is arranged to backreflect the second beam portion and generate a backreflected beam that travels in a second direction opposite to the first direction, A confocal optical pulse stretcher, wherein the beam splitter is further arranged to divide the back-reflected beam into a third beam portion that passes through the segment and is incident on the second concave mirror.

2. The confocal optical pulse stretcher according to claim 1, wherein the reflective element is directed at a certain angle with respect to the back-reflected light passing through the beam splitter.

3. The confocal optical pulse stretcher according to claim 1, wherein the reflective element has a reflective surface arranged to redirect the back-reflected light.

4. A device for facilitating the alignment of a beam splitter in a confocal optical pulse stretcher, The confocal optical pulse stretcher includes at least a first concave mirror and a second concave mirror that is confocal with the first concave mirror. The confocal optical pulse stretcher further includes a retroreflector, The beam splitter is optically positioned between the first concave mirror and the second concave mirror. The beam splitter is further arranged to divide the first beam into a first beam portion which is guided to the first concave mirror and a second beam portion which advances toward the retroreflector in the first direction. The retroreflector is arranged to backreflect the second beam portion and generate a backreflected beam, The first beam portion passes through the optical path in the confocal optical pulse stretcher, The optical path includes the segment between the second concave mirror and the beam splitter. The beam splitter is further arranged to divide the back-reflected beam into a first portion that proceeds to the second concave mirror and a second portion that passes through the beam splitter. The aforementioned device is A reflecting element having an aperture that receives the second portion of the back-reflected beam light passing through the beam splitter and is arranged to redirect the back-reflected beam light, A fluorescent screen arranged to receive the light whose direction has been changed, A device equipped with the following features.

5. The apparatus according to claim 4, wherein the reflective element is directed at a certain angle to the second portion of the back-reflected beam light passing through the beam splitter.

6. The apparatus according to claim 4, wherein the reflective element has a reflective surface arranged to redirect the second portion of the back-reflected beam light.

7. A method for aligning a beam splitter in a confocal optical pulse stretcher, The confocal optical pulse stretcher includes at least a first concave mirror and a second concave mirror that is confocal with the first concave mirror. The beam splitter is optically positioned between a first spot on the first concave mirror and a second spot on the second concave mirror. The beam splitter is further arranged to divide the first incoming beam, which is traveling in the first direction, into a first beam portion which is guided to the first spot and a second beam portion which continues to travel toward the retroreflector in the first direction. The first beam portion passes through the optical path in the confocal optical pulse stretcher, The optical path includes the segment between the second spot and the beam splitter. The first beam portion strikes the second spot of the second concave mirror, The aforementioned method, The second beam portion is reflected back to the beam splitter along a return path in a second direction opposite to the first direction, The second beam portion that has been back-reflected is divided into a beam that travels from the beam splitter to the second spot and a third beam portion that continues to travel in the second direction. The divided, back-reflected second beam portion is positioned to incident on the second spot on the second concave mirror, Methods that include...

8. The third beam portion is directed onto a fluorescent screen to generate an image, Aligning the retroreflector based at least partially on determining whether the image has predetermined characteristics, The method according to claim 7, further comprising:

9. The method according to claim 8, wherein generating an image by directing a portion of the third beam portion onto a fluorescent screen includes reflecting at least a portion of the third beam portion onto the fluorescent screen using a reflective element having an aperture.

10. The method according to claim 9, wherein determining whether the image has predetermined features includes determining whether the image includes light reflected symmetrically from the periphery of the aperture.

Images